The overall aim of these studies is to determine to what extent functional heterogeneity of adenohypophyseal prolactin secreting cells (lactotropes) explains the mechanism of action of hypothalamic, hypophysiotropic agents (TRH and dopamine), and whether functional heterogeneity can be related to the several morphologic sub-types of lactotropes that have been described. To accomplish this aim, we have developed a method that allows the identification of individual cells that secrete prolactin. This method is known as the reverse hemolytic plaque assay. Prolactin secreting cells in culture form plaques (zones of hemolysis around the lactotropes) when incubated for a short time with Staphylococcal protein A-coated erythrocytes, with prolactin antiserum, and with complement. The size of the plaque is related to the amount of prolactin secreted and the % adenohypophyseal cells showing plaques is the % lactotropes in the culture. TRH increases and dopamine decreases, the size and number of plaques formed. Using this basic method together with immunocytochemistry, electron microscopy, autoradiography, and sequential plaque assays on identified lactotropes, I will test the following specific aims: 1) to study Prl secretion by individual lactotropes obtained from animals in various physiologic states and their secretory responses to hypophysiotropic agents, 2) to investigate the possibility that subsets of lactotropes respond to some secretagogues but not others, 3) to identify the subset of lactotropes purported to release first the most recently synthesized prolactin and to test the hypothesis that this lactotrope is unresponsive to TRH but preferentially responsive to dopamine, and 4) to study the electron microscopic morphology of subsets of lactotropes identified in the foregoing sets of experiments. The scientific disciplines involved are physiology, biochemistry, cell biology, immunology, and anatomy. The findings of these studies will contribute to an understanding of the etiology of pituitary adenomas.